The present invention is related to liquid dispensing systems. In particular, the present invention relates to airless sprayers for dispensing paints, varnishes and the like.
Paint sprayers are well known and popular for use in painting of surfaces, such as on architectural structures, furniture and the like. Airless paint sprayers provide a high quality finish due to their ability to finely atomize liquid paint. These airless paint sprayers are typically coupled to a paint source, include a pumping mechanism that draws in the paint, and include a small, shaped orifice through which the paint is discharged. The pumping mechanisms are typically driven by an electric motor, which is operator actuated by a trigger. Airless paint sprayers are capable of pressurizing liquid paint to upwards of 3,000 psi [pounds per square inch] (˜20.7 MPa). Due to these high pressures, paint sprayers often include a relief valve positioned between the pumping mechanism and the discharge orifice.
A conventional relief valve comprises a simple spring-biased valve that opens at an overpressure condition. Additionally, the valve can be manually actuated to prime the pumping mechanism and to relieve pressure and drain paint after operation is completed.
Some airless sprayers have complex, separate pressure control devices, such as an electronic transducer, a bourdon tube, or a spring-loaded pressure-actuated piston, to activate an electrical switch to turn on/off the motor. For example, fluid delivery systems have been outfitted with Bourdon tubes to provide a visual indication of pressure. In other designs, a bourdon tube, or other pressure transducer, is provided that automatically turns the drive motor off when a threshold pressure level is exceeded, such as described in U.S. Pat. No. 5,292,232 to Krohn et al, which is assigned to Graco Inc. Spraying is thus interrupted while pressure within the system rebalances as the motor turns off and on, resulting in varying system pressure, potentially diminishing the quality of the sprayed finish. There is, therefore, a need for improving control over spray parameters in airless sprayers; in particular, one providing essentially continuous control of pressure without costly and/or complex components, and light in weight for hand-held applications.